RSA非對稱加密解密Java實例代碼

String content = "";
for(int i=1;i<100;i++)content += i + ") Easy to say, hard to do.\n";

/1、利用公鑰加密至文件/
{
    /1）根據公鑰反向構造PublickKey對象，調用cipher的init方法時使用/
    String KEY_PUBLIC  = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAIv38xXk06As11OCWq0IPQoQAmz20ZiE8T5KeaMTUbhzUkGNTAQygApua71R/INeEDNsyyQS4PT6EaWTjJop2rcCAwEAAQ==";
    X509EncodedKeySpec keySpec = new X509EncodedKeySpec(new BASE64Decoder().decodeBuffer(KEY_PUBLIC));
    PublicKey key = KeyFactory.getInstance("RSA").generatePublic(keySpec);

Cipher cipher = Cipher.getInstance("RSA");//RSA/ECB/PKCS1PADDING
cipher.init(Cipher.ENCRYPT_MODE, key);

/*
 * 直接加密，因為內容太長將有異常，需要分塊加密
 * javax.crypto.IllegalBlockSizeException: Data must not be longer than 53 bytes
 * cipher.doFinal(content.getBytes());
 */

/*2）分塊加密，與KeyPairGenerator.initialize(int keysize)有關，本例使用的keysize為512*/
int blockSize = 53;//cipher.getBlockSize()為0？根據異常提示設的53
byte[] encryptByte = content.getBytes();
//根據塊大小分塊，不足一塊的部分為一塊
int blocksNum = (int)Math.ceil((1.0*encryptByte.length)/blockSize);

FileOutputStream out = new FileOutputStream("license.lic");
for (int i = 0; i < blocksNum; i++) {
    if (i < blocksNum - 1) {
        out.write(cipher.doFinal(encryptByte, i * blockSize, blockSize));
    } else {
        out.write(cipher.doFinal(encryptByte, i * blockSize, encryptByte.length - i * blockSize));
    }
}

out.close();

}
/2、利用私鑰從文件解密/
{
    /1）根據私鑰反向構造PrivateKey對象/
    String KEY_PRIVATE = "MIIBVAIBADANBgkqhkiG9w0BAQEFAASCAT4wggE6AgEAAkEAi/fzFeTToCzXU4JarQg9ChACbPbRmITxPkp5oxNRuHNSQY1MBDKACm5rvVH8g14QM2zLJBLg9PoRpZOMminatwIDAQABAkBqDAR7FBJbC15hgSQecV194D9WO3L18dOt9FNQgPSroGVYIvIizp1/wIHpMTN6uHwSoaZQcOqV33gxLF6fKbwxAiEAv390Q3X2cBjeScxhNbqPiOspE9rYD3eWSajN6Q7ud7UCIQC7HTUZelHMCpv4xPzg6e1QZkWhBfuqkhg9aOeAnIW0OwIgLW5Tat3FhXqg4ek29sQ34UfJCwjUUXcRlJATqcL9GDECIQChqe+JzrxDbVsrCY9vB83JLEO2hwPUcJtO24dBAHsopwIgEV547YcgZ+pyI1dnQhiLJiiFif+h1aBzaIH5mrkshtw=";
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(new BASE64Decoder().decodeBuffer(KEY_PRIVATE));
    PrivateKey key = KeyFactory.getInstance("RSA").generatePrivate(keySpec);

Cipher cipher = Cipher.getInstance("RSA");//RSA/ECB/PKCS1PADDING
cipher.init(Cipher.DECRYPT_MODE, key);

/*2）分塊解密，根據每塊加密輸出大小確定*/
FileInputStream cis = new FileInputStream("license.lic");
ByteArrayOutputStream bos = new ByteArrayOutputStream();
/*加密時分塊加密，加密輸出結果大小固定*/
byte[] data = new byte[cipher.getOutputSize(cis.available())];
int len = 0;
while((len = cis.read(data)) > 0){
    bos.write(cipher.doFinal(data, 0, len));
}
cis.close();

System.out.print(new String(bos.toByteArray()));

}</pre>